Abbreviations used in this description and/or in the referenced drawings are defined below following the Detailed Description section.
Inter-frequency mobility procedures for LTE have been designed primarily so that UE remains in coverage. With the deployment of heterogeneous network (hetnet) network elements such as pico-cells, slightly different procedures for inter-frequency measurements could be considered which would better meet the needs of handover to capacity hotspots.
FIG. 1 illustrates a hetnet showing one possible hotspot scenario, specifically scenario 4 for carrier aggregation detailed at 3GPP TS 36.300v11.0.0 informative annex J.1. The eNB 101 provides macro coverage on frequency f1 110 and a number of remote radio heads 102a, 102b are deployed on frequency f2 120 in areas of high traffic such as railway stations, airports, shopping malls, etc. Alternatively, dedicated pico eNBs may be deployed in place of the remote radio heads to provide the hotspot coverage.
With this scenario in mind, now consider the conventional neighbour cell measurement procedures in LTE. The network provides to the UE in the MeasConfig information element a parameter s-Measure which is a threshold for the PCell (macro cell in FIG. 1) that controls whether or not the UE is required to perform neighbour cell measurements. If the serving cell reference signal received power RSRP measured by the UE is greater than the s-Measure parameter, the UE does not measure other cells. There is only one value for s-Measure at any given time for a UE, meaning that same value controls intra-frequency, inter-frequency and inter-RAT measurements. The network can disable this by setting the value “0” for the s-Measure parameter.
The assumption behind this conventional use of the s-Measure parameter is that if the serving cell is good enough, no neighbour cells need to be measured. This makes sense for a coverage-based handover. But the hetnet scenario of FIG. 1 contemplates also capacity-based handovers to a hotspot which may be located within an area of strong macro coverage. In a capacity-based handover, the macro cell 101 is not handing over to assure continuous coverage for the UE as is typical if the RSRP is trending lower, but in order to offload the UE's traffic from the macro cell's f1 frequency layer 110 to the pico cell's f2 frequency layer 120 due to congestion on the macro cell's f1 frequency layer 110. Such a capacity-based handover may be to a hotspot which is located within an area of strong macro cell coverage, and the conventional s-Measure parameter is not very effective for this purpose. The only way in current specifications to ensure that the UE searches for pico cell hotspots 120 that may continuously lie in good macro cell coverage 110 would be to disable s-Measure. One exemplary disadvantage of this technique is that disabling the s-Measure parameter precludes its use for intra-frequency and inter-RAT measurements as well as for any coverage based inter-frequency measurements.
Inter-frequency measurements, either with or without measurement gaps, are assumed to be shared equally between all configured inter-frequency/inter-RAT component carriers, and the current measurement performance requirements are strict since the intention is to avoid dropping a call if the UE reaches the edge of coverage on its current serving frequency/frequencies.
Co-owned U.S. patent application Ser. No. 13/023,675 concerns measuring inter-frequency and inter-RAT neighbour cells and describes that the UE's measurement occasions are used for different neighbour cells depending on whether the UE has good or marginal coverage with its serving cell. It describes a priority re-selection algorithm in the E-UTRAN system by which the network can prioritize measurements of either a frequency layer or a RAT over another, so as to more efficiently use the available measurement occasions depending on the strength of the serving cell.
Co-owned U.S. patent application Ser. No. 13/251,363 detail selecting different sets of entries from the prioritized neighbour cell list depending on whether the UE is searching for purposes of maintaining coverage or for obtaining enhanced services, and the coverage/services distinction is determined at least in part based on the UE's received signal strength or signal quality of its serving cell.
What is needed in the art is a way to distinguish when the UE should do coverage-based neighbour cell measurements and when it should do capacity-based neighbour cell measurements.